In a hybrid electric vehicle (HEV), either or both of an internal combustion engine and an electric motor are capable of supplying power to the wheels of the vehicle. Various architecture of HEVs are known. For example, in a “series” hybrid drive train, there is no mechanical coupling of the engine to the wheels. Instead, the engine acts as a power generating unit, and its energy output is converted into electric energy stored in a battery for use by a main traction motor. In a “parallel” hybrid drive train, the engine can be selectively coupled to the wheels, as can the traction motor. Either or both of the engine and motor can provide propulsion power to the wheels. Other hybrid architectures are known, such as “series-parallel” hybrids.
Hybrid vehicles are desirable for increased fuel efficiencies. To better conserve fuel, processors in the vehicle are specifically programmed to stop or “pull-down” the engine during times that the engine is not needed to propel the vehicle. For example, the engine can be pulled-down and kept off when the power demands from the driver are relatively minimal such that the motor can fulfill all propulsion power demands. When the driver power demand increases such that the electric motor cannot provide enough power to meet the demand, the engine may be activated or “pulled-up” to fulfill the power demand.
Excessive stops and starts of the engine can lead to reduced comfort and drivability perceived by the occupants of the vehicle, as well as reduced fuel economy. When power demands change often and abruptly during a drive, the engine may start and stop an undesirable amount of times. Control strategies are known in the art to use filters or other algorithms to “learn” the driving habits of a driver and correspondingly reduce the frequencies of engine starts and stops.